#include<iostream>
using namespace std;
int k = 0;
int main()
{
	int tmp = 1;int cnt = 1;
	int ret = 0;
	cin >> k;
	for (int i = 1;i <= k;i++)
	{
		ret += tmp;
		cnt--;
		if (cnt == 0)
		{
			tmp++;
			cnt = tmp;
		}
	}
	cout << ret << endl;
	return 0;
}